Phase change memory (PCM) devices are positioned to replace traditional types of dynamic random access memory (DRAM) and are currently deposited through the use of physical vapor deposition methods, such as sputtering. PCM devices are fast approaching their physical limits with regard to speed and density within planar structures. Therefore, there is an increasing need to deposit films that allow deposition onto non-planar structures, thereby achieving higher density and speed within the device. Germanium telluride based materials are the basis for PCM devices and are of the general formula GexTey. When antimony is included as a dopant in the material, the general formula is GexSbyTez.
By adding nitrogen to PVD sputtering processes, one can provide for the incorporation of nitrogen into GexSbyTez films with concentrations of up to 15% nitrogen. These nitrogen-incorporated films, in general, have crystallization temperatures that are at least about 50 degrees greater than the crystallization temperatures of GexSbyTez films without nitrogen incorporation. Such nitrogen-incorporated films also have lower crystalline phase conductivities when compared to the GexSbyTez films not having nitrogen. Furthermore, these films generally crystallize at temperatures as low as 200 degrees C., while the desired deposition temperatures for achieving amorphous as-deposited films by CVD or atomic layer deposition (ALD) processes, may be much higher. By depositing such films below the film crystallization temperature, one can produce amorphous thin-films without the typical roughness, poor conformality and/or poor filling of small hole structures (vias), associated with as-deposited crystalline PCM films. Therefore, the direct deposition of amorphous PCM films is highly advantageous.
Some of the nitrogen of the deposited films may be in the form of GeN. In a CVD or ALD process, a portion of the nitrogen not in GeN form is out-diffused or expelled during the post CVD or ALD deposition annealing, depending on the annealing temperature. The annealing facilitates either a phase transformation to the crystalline phase or in keeping the film in the amorphous phase (annealing at a temperature higher than the phase change temperature causes crystallization, and annealing at a temperature lower than the phase change temperature allows the film to remain amorphous). Keeping the film in the amorphous phase leads to the formation of GexSbyTez films having relatively low nitrogen content, such films exhibiting similar structures and device behaviors to that of known GexSbyTez films deposited by PVD. While low amounts of nitrogen are desirable in such films, suitably conformal amorphous films are difficult or impossible to produce without higher amounts of nitrogen to help raise the phase change temperature in CVD or ALD deposition processes.
Making the Ge:Te ratio less than the ideal 1:1 ratio (for stoichiometric GeTe) will also increase the crystallization temperature. This is because an imbalance in the components resulting in excess amounts of Ge will cause the GeTe to act as if there are extrinsic materials that add friction to the crystallization process, thereby increasing the crystallization temperature. Adding nitrogen alone to a GexSbyTez film is not sufficient to increase the crystallization temperature of the GexSbyTez film; however, the reaction of the excess Ge (due to the imbalance between the Ge and Te) with nitrogen in the post deposition annealing leads to more GeN formation and thus purposely promotes the balance of the Ge:Te. Accordingly, the crystallization temperature can then be reduced when compared to an as-deposited film, before annealing.